The differential device of an automobile or the transfer case of a four-wheel drive vehicle normally comprises a pinion shaft with a pinion gear provided on one end section, and this pinion shaft is supported inside a housing by way of a pair of rolling bearings that are separated from each other in the axial direction so as to be able to rotate freely and so as to be able to support axial loads in both directions.
FIG. 9 illustrates a differential device in which a first example of a rotation support device for a pinion shaft having conventional construction is assembled as disclosed in JP 11-048805(A). A differential device is located in the middle of a power transmission line of an automobile so as to reduce the speed of rotation of the propeller shaft as well as to convert the direction of rotation orthogonally. In this differential device, a pair of ring-shaped walls 2a, 2b are located in the portion near the front of the inside of the case 1, which is the housing, such that they are separated in the front/rear direction, and the pinion shaft 3 is supported on the inside of both of these ring-shaped walls 2a, 2b by a pair of single-row tapered roller bearings 4a, 4b so as to be able to rotate freely and be able to support axial loads in both directions. The single-row tapered roller bearings 4a, 4b are constructed such that a plurality of tapered rollers 7a, 7b are located between outer rings (cups) 5a, 5b that fit around the inside of the ring-shaped walls 2a, 2b and inner rings (cones) 6a, 6b that fit around the outside of the pinion shaft 3 so as to be able to roll freely. These single-row tapered roller bearings 4a, 4b are arranged in a back-to-back such that the directions of the contact angles are opposite from each other, so as to support axial loads that are applied to the pinion shaft 3 in both directions (both the left and right direction in FIG. 9). The front/rear relationship in this specification is defined as the front and rear of the vehicle. That is, in FIG. 9, the right side is the “front side”, and the left side is the “rear side”.
A ring-shaped connecting member 8 is fastened around the outside on the front end section of the pinion shaft 3. A connecting flange 9 that constitutes a front end section of this connecting member 8 is located on the portion that protrudes toward the outside from the opening section on the front end of the case 1. The rear end section of the propeller shaft (not illustrated in the figure) is connected to the connecting flange 9. On the other hand, a pinion gear 10 is fastened to the rear end section of the pinion shaft 3, and the pinion gear 10 and a ring gear 11 engage together. The ring gear 11 is supported in the rear section on the inside of the case 1 so as to only be able to rotate freely.
In recent years, there is a strong demand for energy conserving automobiles, so for a pair of rolling bearings that are assembled in a differential device or transfer case such as described above as well, there is a demand to reduce to the dynamic torque (rotation resistance) and starting torque in order to keep the amount of power transmission loss low. The first example of conventional construction is not always able to sufficiently reduce these kinds of torque, and thus is not able to meet this demand.
In other words, in the first example of conventional construction, single-row tapered roller bearings 4a, 4b are used as the pair of rolling bearings assembled in the rotation support device for the pinion shaft. In both of these single-row tapered roller bearings 4a, 4b, in addition to the outer rings 5a, 5b and the inner races 6a, 6b coming in rolling contact with the tapered rollers 7a, 7b, a rib section 12 that exists on the end section on the large-diameter side of the outer circumferential surface of the inner ring 6a, 6b and the head sections of the tapered rollers 7a, 7b (surfaces on the ends on large diameter sides) are always in sliding contact, so it becomes easy for the torque to become large.
JP 4,250,952(B2) discloses construction where by devising the construction of the pair of rolling bearings that are assembled in the rotation support device for a pinion shaft, the torque in both of these rolling bearings is kept small. FIG. 10 illustrates a second example of conventional construction of a rotation support device for a pinion shaft which is disclosed in JP 4,250,952(B2). In this second example of conventional construction, a tandem double-row angular contact ball bearing 13 is used as the rolling bearing on the pinion gear side (left side in FIG. 10) that supports a comparatively large radial load. This tandem double-row angular contact ball bearing 13 is able to support an axial load that acts on the pinion shaft 3 in a direction toward the side opposite the pinion gear from the pinion gear side. On the other hand, a single-row angular contact ball bearing 14 is used as the rolling bearing on the opposite side from the pinion gear side (right side in FIG. 10) that supports a comparatively small radial load. This single-row angular contact ball bearing 14 is able to support an axial load that acts on the pinion shaft 3 in a direction toward the pinion gear side from the side opposite from the pinion gear side.
The tandem double-row angular contact ball bearing 13 that is located on the pinion gear side is constructed by arranging a plurality of balls 19 for each row between the angular double-row outer raceways 16a, 16b that are formed around the inner circumferential surface of the outer ring 15 and the angular double-row inner raceways 18a, 18b that are formed around the outer circumferential surface of the inner ring 17 so that contact angles are applied in the same direction by both rows of balls 20a, 20b, and so that the pitch circle diameter of the ball row 20a on the pinion gear side is greater than the pitch circle diameter of the ball row 20b on the side opposite from the pinion gear side. On the other hand, the single-row angular contact ball bearing 14 that is located on the side opposite from the pinion gear side is constructed by arranging a plurality of balls 25 between the angular outer raceway 22 that is formed around the inner circumferential surface of the outer ring 21 and the angular inner raceway 24 that is formed around the outer circumferential surface of the inner ring 23, so that these balls 25 can roll freely when a contact angle given.
Moreover, in the case of a second example of conventional construction, the diameter of the balls 25 of the ball row of the single-row angular contact ball bearing 14 that is located on the opposite side from the pinion gear side is larger than the diameter of the balls 19 of the ball rows 20a, 20b of the tandem double-row angular contact ball bearing 13 that is located on the pinion gear side, so it is possible to sufficiently support axial loads from both directions. In other words, when the diameter of the balls 19 is taken to be Bd19, and the diameter of the balls 25 is taken to be Bd25, Bd19<Bd25. However, when necessary, it is possible to employ construction wherein the diameter of the balls 19 of the ball rows 20a, 20b of the tandem double-row angular contact ball bearing 13 is greater than the diameter of the balls 25 of the ball row of the singular angular contact ball bearing 14 (Bd19>Bd25).
In the case of the second example of conventional construction having this kind of construction, a tandem double-row angular contact ball bearing 13 and single-row angular contact ball bearing 14 are used as the pair of rolling bearings for supporting the pinion shaft 3 so as to be able to freely rotate, so, it is possible to lower the dynamic torque during operation, as well as lower the starting torque when starting operation, more than in the case of using a pair of single-row tapered roller bearings 4a, 4b as in the first example of conventional construction. However, in the case of the second example of conventional construction as well, from the aspect of being able to further lower the dynamic torque and starting torque, it cannot be said that enough investigation has been performed, and there is still room for improvement.
That is, in the second example of conventional construction, in order to prevent the occurrence of damage such as indentation of the outer raceway 22 and inner raceway 24 of the single-row angular contact ball bearing 14 that is located on the opposite side from the pinion gear side, when the radius of curvature of the cross-sectional shape of the outer raceway 22 is taken to be Ro, the radius of curvature of the cross-sectional shape of the inner raceway 24 is taken to be Ri, and the diameter of the balls 25 is taken to be Bd, the outer raceway groove R ratio, which is the ratio of the radius of curvature Ro and the diameter Bd (Ro/Bd), is regulated within the range 0.510≦Ro/Bd≦0.520, and the inner raceway groove R ratio, which is the ratio of the radius of curvature Ri and the diameter Bd (Ri/Bd), is regulated within the range 0.502≦Ri/Bd≦0.512. However, when the outer raceway groove R ratio and inner raceway groove R ratio are regulated within these ranges, the contact area between each of the outer raceway 22 and inner raceway 24 and the balls 25 becomes too large, and it becomes impossible to sufficiently reduce the dynamic torque during operation.
Furthermore, in this second example of conventional construction, in order to maintain the load capacity of an axial load on the single-row angular contact ball bearing 14 that is located on the opposite side from the pinion gear, when the contact angle between each of the outer raceway 22 and inner raceway 24 and the balls 25 is taken to be a, the value of this contact angle α is regulated within the range 30° to 45°. However, when the contact angle α is regulated within such a range, the value of the contact angle α becomes too small, and there is a possibility that it will become impossible to sufficiently reduce the starting torque at the start of operation.
Therefore, in the case of the second example of conventional construction, it cannot be said that enough investigation has been performed in order to be able to further reduce the dynamic torque during operation or the starting torque at the start of operation, and there is still room for improvement. In order to reduce the dynamic torque during operation, it is considered possible to increase the outer raceway groove R ratio and the inner raceway groove R ratio to values greater than in the ranges described above, however, in that case, there is a possibility that the axial rigidity of the single-row angular contact ball bearing will be insufficient, and thus there is the possibility of new problems such as noise occurring in the area of engagement between the pinion gear and the ring gear.
In addition, JP 4,058,241(B2) discloses using a tandem double-row angular contact ball bearing on both the pinion gear side and the side opposite from the pinion gear side as the pair of rolling bearings assembled in the rotation support device for a pinion shaft. In this construction, compared with the case of using a single-row angular contact ball bearing as the rolling bearing on the opposite side from the pinion gear side, it is possible to increase the load capacity, however, the dynamic torque (rotation resistance) increases, so it becomes difficult to meet the demand for lower fuel consumption of the vehicle.